There is currently considerable interest in the development of processes and apparatus to enable the manufacture of polymer filaments, fibres, ribbons or sheets. It is theoretically possible to obtain materials with high tensile strength and toughness by engineering the orientation of the polymer molecules and the way in which they interact with one another. Strong, tough filaments, fibres or ribbons are useful in their own right for the manufacture, for example, of sutures, threads, cords, ropes, wound or woven materials. They can also be incorporated into a matrix with or without other filler particles to produce tough and resilient composite materials. Sheets whether formed from fibres or ribbons can be stuck together to form tough laminated composites.
Natural silks are fine, lustrous filaments produced by the silk-worm Bombyx mori and other invertebrate species. They offer advantages compared with the synthetic polymers currently used for the manufacture of materials. The tensile strength and toughness of the dragline silks of certain spiders can exceed that of Kevlar™, the toughest and strongest man-made fibre. Spider dragline silks also possess high thermal stability. Many silks are also biodegradable and do not persist in the environment. They are recyclable and are produced by a highly efficient low pressure and low temperature process using only water as a solvent. The natural spinning process is remarkable in that an aqueous solution of protein is converted into a tough and highly insoluble material.
According to an article by J. Magoshi, Y. Magoshi, M. A. Becker and S. Nakamura entitled “Biospinning (Silk Fiber Formation, Multiple Spinning Mechanisms)” published in Polymeric Materials Encyclopedia, by the Chemical Rubber Company, it is reported that natural silks are produced by sophisticated spinning techniques which cannot yet be duplicated by man-made spinning technologies.
Other known processes for producing filaments from liquid raw material are disclosed in GB-A-441440 and U.S. Pat. No. 2,450,457. These known processes pass the liquid raw material through rigid porous tubes.
Fibres produced by existing technological processes and apparatus suffer from the following disadvantages. Many show “die swell” which leads to some loss of molecular orientation with a consequent degradation of mechanical properties. This is not seen in natural silks which show strongly uniaxial orientation. Furthermore, existing processes are not energy efficient, requiring high temperatures and pressures to reduce the viscosity of the feedstock so that it can be forced through a die. Separate stages are often required, for example for further “draw-down”, to anneal the fibre with heat, and to process it through separate acid or alkaline treatment baths.
Disclosure of the Invention
It is an aim of the present invention to provide an improved method and apparatus for spinning a liquid spinning solution or “dope”.
According to a first aspect of the invention there is provided spinning apparatus for forming spun material from a liquid spinning solution, the apparatus including a die assembly having at least one tubular passage through which the liquid spinning solution is passed having at least partly permeable walls, wherein said walls defining the or each tubular passage comprise at least one semipermeable membrane and/or at least one porous membrane. Preferably enclosure means surround the walls. The provision of enclosure means allows components of fluent material contained in the enclosure means and in contact with the walls to pass through the or each semipermeable or porous membrane. Alternatively components of the liquid spinning solution passing through the or each tubular passage may pass outwardly through the walls of the semipermeable or porous membrane. In addition, since the or each semipermeable or porous membrane is generally flexible, it will be necessary to fill the enclosure means with a pressurised fluent material to maintain the shape of the walls defining the tubular passage during passage of the spinning solution through the tubular passage.
According to a second aspect of the invention there is provided a method of forming material by passing liquid spinning solution through at least one tubular passage of a die assembly having at least partly prmeable walls, wherein the walls of the or each tubular passage comprise at least one semipermeable membrane and/or at least one porous membrane and in that the liquid spinning solution is treated, as it passes along the or each tubular passage, by components permeating through the semipermeable or porous membrane of said walls. In this way fluent material may pass inwardly into, or outwardly from, the or each tubular passage through their semipermeable or porous walls.
The discovery of the way in which spiders produce dragline silk provides the basis for the invention. We have found that by making the walls of the or each tubular passage at least partly permeable or porous, preferably selectively permeable along the length of the tubular passage, which is preferably tapered, it is possible to control properties such as the pH, water content, ionic composition and shear regime of the spinning solution in different regions of the tubular passage of the die. Ideally this enables the phase diagram of the spinning solution to be controlled allowing for pre-orientation of the fibre-forming molecules followed by a shear-induced phase separation and allowing the formation of insoluble fibres containing well-orientated fibre-forming molecules.
Conveniently the walls defining the tubular passage(s) are surrounded by said enclosure means to provide one or more compartments. These compartments act as jackets around the tubular passage(s). The or each tubular passage suitably has an inlet at one end to receive the spinning solution and an outlet at the other for the formed or extruded material and is typically divided into three parts arranged consecutively, the first part allowing for the pre-treatment and pre-orientation of the fibre-forming polymer molecules in the liquid feedstock prior to forming the material by draw down, the second region in which draw down of the “thread” takes place and which functions as a treatment and coating bath, and the third part has an outlet or opening of restricted cross-section which serves to prevent the loss of the contents of the “treatment bath” with the emerging fibre and to provide for the commencement of an optional air drawing stage.
It will be appreciated that any solution or solvent or other phase or phases surrounding the fibre in the second part of the or each tubular passage also serves to lubricate the fibre as it moves through and out of the tubular passage.
All or part of the length of each tubular passage typically has a convergent geometry typically with the diameter decreasing in a substantially hyperbolic fashion. According to G. Y. Chen, J. A. Cuculo and P. A. Tucker in an article entitled “Characteristic and Design Procedure of Hyperbolic Dies” in the Journal of Polymer Sciences: Part B: Polymer Physics, Vol 30, 557-561 in 1992, it is reported that the orientation of molecules in a fibre can be improved by using a die with a convergent hyperbolic geometry instead of the more usual parallel capillary or conical dies.
The geometry of substantially all or part of the or each tubular passage may be varied to optimise the rate of elongational flow in the spinning solution (dope) and to vary the cross-sectional shape of the formed material produced from it. The preferred substantially hyperbolic taper for part or all of the or each tubular passage maintains a slow and substantially constant elongational flow rate thus preventing unwanted disorientation of the fibre-forming molecules resulting from variation in the elongational flow rate or from premature formation of insoluble material before the dope has been appropriately preoriented. A convergent taper to the tubular passage of the die will induce elongational flow which will tend to induce a substantially axial alignment in the fibre-forming molecules, short fibres or filler particles contained in the dope by exploiting the well known principle of elongational flow. Alternatively, the principle of elongational flow through a divergent instead of convergent die can be used to induce orientation in the hoop direction that is approximately transverse to the longitudinal axis of the extruded material.
The diameter of the or each tubular passage may be varied to produce fibres of the desired diameter.
The rheology of the liquid feedstock in the tubular passage of the die is largely independent of scale enabling the size of the apparatus to be scaled up or down. The convergence of the tubular passage allows a wide range of drawing rates to be used typically ranging from 0.01 to 1000 mm sec−1. If fibres are being extruded they may typically have a diameter of from 0.1 to 100 μm. Typically the outlet of the tubular passage has a diameter of from 1 to 100 μm with the diameter of the inlet of the tubular passage being from 25 to 150 times greater depending on the extensional flow it is desired to produce. Tubular passages with a circular cross-section are used to produce fibres with circular cross sections. Tubular passages of alternative cross-sectional shapes can be used to produce fibres, flat ribbons or sheets of extruded materials with other cross-sectional shapes.
All or part or parts of the walls of the or each tubular passage of the die assembly are constructed from or formed or moulded from selectively permeable or porous membranes, such as cellulose acetate-based membrane sheets. The membrane can be substituted with diethylaminoethyl or carboxyl or carboxymethyl groups to help maintain protein-containing dopes in a state suitable for spinning. Other examples of permeable or porous membranes are hollow-fibre membranes, such as hollow fibres constructed from polysulfone, polyethyleneoxide-polysulfone blends, silicone or polyacrylonitrile. The exclusion limit selected for the semipermeable membrane will depend on the size of the small molecular weight constituents of the dope but is typically less than 12 kDa.
All or part of the walls of the or each tubular passage can he constructed from selectively permeable or porous membranes in a number of different ways. By way of example only a selectively permeable or porous sheet can be held in place over a groove with suitable geometry cut into a piece of material to form the tubular passage. Alternatively two selectively permeable or porous membranes can be held in place on either side of a separator to construct the tubular passage. Alternatively a single sheet can be bent round to form a tubular passage. A hollow tube of selectively permeable or porous membrane material(s) can also be used to construct all or part of the tubular passage. By way of example only, a variety of methods are available to shape the tube into a die as is commonly known to a craftsman skilled in the art.
The use of selectively permeable or porous walls for substantially all or part or parts of the tubular passage(s) enables the proper control within desired limits of, for example, the concentration of fibre-forming material; solute composition; ionic composition; pH; dielectric properties; osmotic potential and other physico chemical properties of the dope within the tubular passage by applying the well-known principles of dialysis, reverse dialysis, ultrafiltration and preevaporation. Electro-osmosis can also be used to control the composition the dope within the tubular passage. It will be appreciated that a control mechanism receiving inputs relating to the product being formed, for example the diameter of the extruded product and/or the resistance countered in the tubular passage, such as during extrusion through the outlet of the tubular passage, can be used to control, for example, polymer concentration, solute composition, ionic composition, pH, dielectric properties, osmotic potential and/or other physicochemical properties of the dope within the tubular passage.
The selective permeability and/or porosity of the walls of the or each tubular passage may also allow for the diffusion through the walls of further substances into the tubular passage(s) provided that these have a molecular weight lower than the exclusion limit of the selectively permeable material from which the walls of the tubular passage(s) are constructed. By way of example only the additional substances added to the dope in this manner may include surfactants; dopants; coating agents; cross-linking agents; hardeners; and plasticisers. Larger sized aggregates can be passed through the walls of the tubular passage if it is porous rather than being simply semipermeable.
The compartments surrounding the walls of the tubular passage or passages may act as one or more treatment zones or baths for conditioning the fibre as it passes through the tubular passage(s). Additional treatment can occur after the material has exited the outlet of the tubular passage.
One or more regions of the or each tubular passage may be surrounded by one or more compartments arranged consecutively so as to act as a jacket or jackets to hold solution, solvent, gas or vapour in contact with the outer surface of the selectively permeable walls of the tubular passage(s). Typically solution, solvent, gas or vapour is circulated through the compartment or compartments. The walls of the compartment or compartments are sealed to the outer surface of the walls of the tubular passage(s) by methods that will be understood by a person skilled in the art. The compartment or compartments serve to control the chemical and physical conditions within the or each tubular passage. Thus the compartments surrounding the tubular passage(s) serve to define the correct processing conditions within the dope at any point along the tubular passage(s) In this way parameters such as the temperature; hydrostatic pressure; concentration of fibre-forming material; pH; solute; ionic composition; dielectric constant; osmolarity or other physical or chemical parameter can be controlled in different regions of the tubular passage as the dope moves down the length of the die. By way of example only, continuously graded or stepped changes in the processing environment can be obtained.
Conveniently a selectively permeable/porous membrane can he used to treat one side of a forming extrusion in a different way to the other side. This can be used, for example, to coat the extrusion or remove solvent from it asymmetrically in such a way that the extrusion can be made to curl or twist.
All or part of the draw down process may typically occur within the die rather than at the outer face of the die assembly as occurs in existing spinning apparatus. The former arrangement offers advantage over existing spinning apparatus. The distortion of molecular alignment due to die swell is avoided. The region of the die assembly after the internal commencement of the draw down taper can be used to apply coatings or treatments to the extrusion. Further, the last part of the die assembly is water lubricated by the solvent-rich phase surrounding the extrusion.
By way of example only the apparatus can be used for forming fibres from dopes containing solutions of recombinant spider silk proteins or analogues or recombinant silk worm silk proteins or analogues or mixtures of such proteins or protein analogues or regenerated silk solution from silkworm silk. When these dopes are used it is necessary to store the dope at a pH value above or below the isoelectric point of the protein to prevent the premature formation of insoluble material. It will be appreciated that other constituents may be added to the dope to keep the proteins or protein analogues in solution. These constituents may then be removed through the semipermeable and/or porous walls when the dope has reached the appropriate portion of the tubular passage in which it is desired to induce the transition from liquid dope to solid product, e.g. thread or fibre. The dope within the tubular passage can then be brought by dialysis against an appropriate acid or base or buffer solution to a pH value at or close to the pK value of one or more of the constituent proteins of the dope. Such a pH change will promote the formation of an insoluble material. A volatile base or acid or buffer can also be diffused through the walls of the or each tubular passage from a vapour phase in the surrounding compartment or jacket to adjust the pH of the dope to the desired value. Vapour phase treatment to adjust the pH can also occur after the extruded material has left the outlet of the die assembly.
The draw rate and length, wall thickness, geometry and material composition of the or each tubular passage may be varied along its length to provide different retention times and treatment conditions to optimise the process.
One or more regions of the walls defining the or each tubular passage can be made impermeable by coating their inner or outer surfaces with a suitable material to modify the internal environment in a length of the tubular passage using any coating method as will be understood by a person skilled in the art.
The inner surface of the walls of the or each tubular passage can be coated with suitable materials to reduce the friction between the walls of the tubular passage and the dope or fibre. Such a coating can also be used to induce appropriate interfacial molecular alignment at the walls of the tubular passage in lyotropic liquid crystalline polymers when these are included in the dope.
A further embodiment allows for one or more additional components to be fed to the start of the or each tubular passage via concentric openings to allow two or more different dopes to be co-extruded through the same tubular passage allowing for the formation of one or more coats or layers to the fibre or fibres.
A further embodiment utilises a dope prepared from a phase separating mixture containing two or more components which, for example, may be different proteins. The removal or addition of components through the selectively permeable and/or porous material can be used to control the phase separation process to produce droplets of one or more components typically with a diameter of 100 to 1000 nm within the bulk phase in the final extrusion. These can be used to enhance the toughness and other mechanical properties of the extrusion. The use of a convergent or divergent die conveniently induces elongational flow in the droplets to produce orientated and elongated filler particles or voids within the bulk phase. A convergent die will orientate and elongate such droplets in a direction parallel to that of the formed product whereas a divergent die will tend to orientate the droplets in hoops transverse to the direction of flow within the tubular passage. Both types of arrangement can be used to enhance the properties of the formed product. Further it will be understood that the selectively permeable or porous walls of the or each tubular passage can be used to diffuse in or out chemicals to initiate the polymerisation of filler particles.
The spinning apparatus with one or more tubular passages surrounded by a compartment or compartments to act as jackets can be constructed by one or two stage moulding or other methods known to a person skilled in the art. It will be appreciated that a moulding process can be used to create simple or complete profiles for the or each tubular passage and the outlet of the die assembly. Very small flexible lips can be formed, e.g. moulded, at the outlet to prevent the escape of the contents of the treatment bath and act as a restriction to enable an optional additional air drawing stage or wet drawing after the material has left the outlet of the die assembly should this be required. The microscopic profile of the inner surface of the lips at the outlet can be used to modify the texture of the surface coating of the extruded material.
By way of example only, the jackets and supports for the tubular passages can be constructed from two or more components formed by injection moulding or constructed in other ways as will be understood by a person skilled in the arts. It will be appreciated that this method of construction is modular and that a number of such modules can be assembled in parallel to produce simultaneously a number of fibres or other shaped products. Sheet materials can be produced by a row or rows of such modules. Such a modular arrangement allows for the use of manifolds to supply dope to the inlet of the tubular passage(s) and to supply and remove processing solvents, solutions, gases or vapours to and from the jacket or jackets surrounding the tubular passages. Additional components may be added if desired. Potential modifications to the arrangements shown will be apparent to persons skilled in the art.
Other methods of constructing spinning apparatus in which the walls of the tubular passages are substantially or partially constructed from semipermeable or porous membrane material will be known by a person skilled in the art. By way of example only these include micro-machining techniques. In addition it will be appreciated that walls of the tubular passages substantially or partially constructed from semipermeable/porous material can be incorporated into other types of spinning apparatus, such as electrospinning apparatus.
The or each tubular passage may be made self-starting and self-cleaning. It will be appreciated that blockage of spinning dies during the commercial production of extruded materials is time-consuming and costly. To overcome this difficulty, the walls of the tubular passage may be constructed from an elastic material sealed into and surrounded by two or more jackets arranged in sequence. The pressure in each of these jackets can be varied independently by methods that will be understood by a craftsman skilled in the art. Pressure changes in the jackets can be used to change the diameter of different regions of the tubular passage in a manner analogous to a peristaltic pump to pump the dope to the outlet to commence the drawing of fibres or to clear a blockage. Thus a decrease in pressure in a jacket towards the outlet end of the tubular passage will dilate the elastic walls of the tubular passage within the jacket. If the pressure is now raised in a second jacket closer to the input end of the tubular passage a region of the walls of the tubular passage running through this jacket will tend to collapse forcing the dope towards the outlet. Alternatively, the pressure in the dope fed to the tubular passage could be increased causing the diameter of the elastic tubular passage walls to increase. It will be appreciated that both methods could be used together or consecutively. With both methods the elasticity of the passage walls enables the diameter of the tubular passage to be increased reducing the resistance to flow. With both methods it is to be noted that increasing the pressure of the dope will also assist in start up and in clearing blockages in the tubular passage. It will also be appreciated by way of example only that the use of rollers such as are used in peristaltic pumps can be used as an alternative means of applying pressure to pump dope to the outlet to commence spinning or to clear a blockage.
The pressure in the sealed compartments surrounding the tubular passage(s) may be controlled to define and modify the geometry of the tubular passage to optimise pinning conditions.
If the or each tubular passage has a convergent or divergent geometry along all or part of its length, filler articles or short fibres included in the dope may be orientated as they flow through the tubular passage by exploiting the well understood principle of elongational flow. It will be understood that the substantially axial orientation of such filler particles or short fibres will be produced by a convergent tubular passage while a divergent one will produce orientation in the hoop direction, that is approximately transverse to the long axis of the extruded material. Both patterns of orientation confer additional useful properties on the fibre. It will be appreciated that a convergent or divergent geometry of all or part of the or each tubular passage will also serve to elongate and orientate small fluid droplets of an additional solvent or solution or other phase or phases or additional unpolymerised polymer or polymers present in the dope as supplied to the tubular passage or arising by a process of phase separation within the dope. The presence of elongated and well orientated narrow inclusions formed by either a convergent or divergent tubular passage can be used to confer additional useful properties to the extruded material.
It will be appreciated that the direct drawing down of a fibre or other formed product from liquid spinning solution within a region of a tubular passage greatly improves the molecular orientation in the final material avoiding the disorientation produced by die swell produced by other methods of forming the final material. It also greatly reduces the pressure required to form material compared with the extrusion of fibre from a conventional restriction die.
The present invention seeks to alleviate some or all of the problems associated with the prior art by providing reliable apparatus and method for manufacturing materials with a highly defined and typically uniaxial molecular orientation from spinning solutions. The use of permeable/porous tubing, preferably selectively permeable/porous tubing, for the construction of the walls of the tubular passage enables a precise control of all parameters of the processing environment. This enables the processing environment to be precisely defined down the length of the tubular passage. Precise control of the processing environment in the tubular passage enables the polymer concentration, molecular configuration and viscosity and other physical properties of the spinning solution to be kept at optimum values at all points along the tubular passage. The convergent geometry with cross-sectional area decreasing non-linearly and preferably hyperbolically in substantially all or the first part of the tubular passage serves to align the molecules axially before the draw down process thus improving the quality of alignment in the final formed product.
The apparatus may be arranged in such a way that two or more fibres are formed in parallel and twisted around each other or crimped or wound onto a former or coated or left uncoated as desired. The fibres can be drawn through a coating bath and subsequently through a convergent die to give rise to a “sea and island” composite material as will be understood by a person skilled in the art. One or more rows of dies or one or more dies with slit or annular openings can be used to form sheet materials.